Project Summary Title: Examining the Role of Peers on Childhood Obesity and Fitness in NYC The current levels of childhood obesity are unacceptable and worrisome. While emerging research suggests that social and peer influence can amplify the impacts of genetic and environmental factors on health behaviors, several important gaps in knowledge and methodology remain. First, the analysis of the impacts of peers on children's weight outcomes and fitness is quite limited. Second, much past work has been shown to be subject to confounding by selection based on observable characteristics (like socioeconomic status) and unobservable characteristics. Third, while little is known about peer effects on children's obesity and physical activity outcomes, even less is known about the temporal dynamics of that relationship. Understanding the impact of peer effects will be crucial towards developing effective policies to combat the childhood obesity epidemic. The overall aim of this proposal is to examine peer effects of childhood obesity and inform real-time policy design through the utilization of the NYC Fitnessgram Dataset, which tracks the weight and fitness outcomes of NYC public school children from 2005-2013. Thus, Specific Aim 1 probes the social processes that play a role in the obesity epidemic by estimating the causal effect of classmate characteristics on body weight and fitness for multiple levels of social networks across cohorts and within schools. Because childhood, and especially adolescence, is a period of both initiation and maintenance of many risky health behaviors, as well as periods of greater sensitivity to peer pressure and influences, Specific Aim 2 explores the temporal dynamics of the relationship between peer effects and obesity, about which little is known. Both aims utilize a powerful, longitudinal dataset and combine expertise from an interdisciplinary group of economists, epidemiologists, and health policy scholars who have the necessary experience with the data and set of research questions to produce new evidence on the consequences peer effects of obesity with the ultimate aim of developing policy responses that can target and alleviate these consequences.